Organic light-emitting diodes have superior characteristics over plasma display panels (PDPs) and inorganic electroluminescent display devices. These characteristics include low driving voltage (e.g., 10V or less), a broad viewing angle, rapid response time, and high contrast. Based on these advantages, organic light-emitting diodes can be used as pixels in graphic displays, television-image displays, and surface light sources. In addition, organic light-emitting diodes can be fabricated on flexible transparent substrates; they can be reduced in thickness and weight, and have good color representation. Therefore, the potential for organic light-emitting diodes to be used in next-generation flat-panel displays (FPDS) has been recognized.
A representative organic light-emitting diode was reported by Gurnee in 1969. However, this organic light-emitting diode suffers from limitations in its applications because of its limited performance. Since Eastman Kodak Co. reported multilayer organic light-emitting diodes in 1987, remarkable progress has been made in the development of organic light-emitting diodes capable of overcoming the problems of devices used in the prior art.
Such organic light-emitting diodes comprise a first electrode as a hole injection electrode (anode), a second electrode as an electron injection electrode (cathode), and an organic light-emitting layer disposed between the anode and the cathode wherein electrons injected from the cathode and holes injected from the anode combine with each other in the organic light-emitting layer to form electron-hole pairs (excitons), and then the excitons fall from the excited state to the ground state and decay to emit light. At this time, the excitons may fall from the excited state to the ground state via the singlet excited state to emit light (i.e. fluorescence), or the excitons may fall from the excited state to the ground state via the triplet excited state to emit light (i.e. phosphorescence). In the case of fluorescence, the probability of the singlet excited state is 25% and thus the luminescence efficiency of the devices is limited. In contrast, phosphorescence can utilize both probabilities of the triplet excited state (75%) and the singlet excited state (25%), and thus the theoretical internal quantum efficiency may reach 100%. Therefore, it is necessary to develop novel phosphorescent compounds suitable for phosphorescent OLEDs to enhance the luminous efficiency.
An OLED is typically categorized as either a micro-molecular OLED or a high-molecular OLED, according to its material type. At present, since micro-molecular OLEDs have a relatively higher efficiency, brightness, and lifetime than high-molecular OLEDs, the use of micro-molecular OLEDs is a trend in the OLED field. A micro-molecular OLED is generally fabricated by way of vacuum evaporation, so that the micro-molecular materials have good film forming qualities. However, 95% of the organic electroluminescent materials are deposited on the chamber wall of the manufacturing equipment used to manufacture the OLED, such that only 5% of the organic electroluminescent materials are coated on a substrate after the manufacturing process, resulting in a high investment cost. Therefore, a wet process (such as spin coating or blade coating) has been provided to fabricate micro-molecular OLEDs to improve the utilization ratio of organic electroluminescent materials and reduce the cost of manufacturing OLEDs. Unfortunately, conventional phosphorescent organic electroluminescent materials are not suitable to be used in the wet process due to their inferior solubility.
Therefore, it is necessary to develop novel phosphorescent organic compounds suitable for use in a wet process to fabricate phosphorescent OLEDs to solve the above problems.